Cm

Figure 2.49. Northward transport of angular momentum averaged over year and longitude. The meridional contribution is the one carried by meridionally averaged winds, whereas the eddy contribution is obtained as the zonal average of the transport arising from the deviations from the zonal average of the wind velocity vector (thus including large-scale, horizontal eddies) (based on Buch, 1954; Obasi, 1963, as quoted by Lorenz, 1967).

Latitude (deq)

Figure 2.49. Northward transport of angular momentum averaged over year and longitude. The meridional contribution is the one carried by meridionally averaged winds, whereas the eddy contribution is obtained as the zonal average of the transport arising from the deviations from the zonal average of the wind velocity vector (thus including large-scale, horizontal eddies) (based on Buch, 1954; Obasi, 1963, as quoted by Lorenz, 1967).

In order to describe all the necessary transport of heat, any accurate model of the circulation of the atmosphere must include couplings to the heat transport within the ocean-continent system (ocean currents, rivers, run-off along the surface and to a lesser extent, as far as heat transport is concerned, ground water motion). Such coupled models will be considered below in section 2.3.2.

The kinetic energy of the atmospheric circulation is diminished by friction, leading to an irreversible transformation of kinetic energy into internal energy (heat). In order to make up for such frictional losses, new kinetic energy must be created in the atmosphere. This can be achieved essentially by two processes, both of which are reversible and may proceed adiabatically (details of the formulation are given in section 2.C). One is the conversion of potential energy into kinetic energy (by gravitational fall), and the other is the transformation of internal energy into kinetic energy by motion across a pressure gradient.

Creation and destruction of kinetic energy

Direct measurements of heat and temperature distributions (cf. Fig. 2.28 and section 2.C) allow an estimate of the generation of internal energy available for possible conversion into kinetic energy. With the use of wind data (e.g. Figs. 2.45-2.47), it is possible to obtain an estimate of the conversion of available energy and large-scale horizontal eddy motion into zonal motion. Also, the available energy may be split into a zonal part corresponding to the use of the zonal mean temperatures and a (large-scale) eddy part derived from the deviations of the temperature from longitudinal average (Lorenz, 1967; Newell et al., 1969). Separate estimates exist for the frictional losses from zonal and eddy motion as a function of height. An attempt to survey the main conversion processes (deriving poorly known quantities, such as the conversion of available eddy energy into kinetic eddy energy, from balance requirement) is given in Fig. 2.50, based on Lorenz (1967) and considering comments by Newell et al. (1969), who discusses the seasonal variations and uncertainties involved in the procedures used.

On an annual and global average basis, the creation of kinetic energy in the form of large-scale motion (V*, w*) amounts to 2.3 W m-2 or 0.7% of the solar radiation at the top of the atmosphere. For consistency, the frictional losses must be of equal magnitude, which is not quite consistent with direct estimates (4-10 W m-2). Newell et al. argue that the value of about 2.3 W m-2 given in Fig. 2.50 is most likely to be correct.